This invention relates to electrostatic spray apparatus, and more particularly to electrostatic spray apparatus including a self-contained energy conversion system adapted to convert the energy of a fluid under pressure into electrical energy for electrostatic spraying.
The type of spray apparatus of concern herein is disclosed in U.S. Pat. Nos. 4,219,865 and 4,290,090, each to Malcolm, and U.S. Pat. No. 4,377,838 to Levey et al. The teachings of each of the foregoing patents and patent application are incorporated herein by reference.
In the prior energy conversion systems, an air motor and a directly driven alternator provide an electrical power signal which is modified by downstream electrical components to provide a final electrical potential suitable for application to the electrode of the spray apparatus. The air motors are impulse-type motors or turbines having a rotor which is driven by an impinging air flow. In accordance with a preselected air flow to the spray apparatus, the air motor and alternator are designed to accelerate in a minimum period of time to a steady state or final running speed such that the electrical power signal from the alternator is suitable for conversion to the required electrostatic spray potential by the downstream electrical components. The period of acceleration should be minimized in order to enable prompt spraying at the full electrostatic potential upon actuation of the spray apparatus.
The design of energy conversion systems to provide proper acceleration and running speed characteristics has comprised heretofore essentially a design trade-off or balancing problem with rapid acceleration being accompanied by high running speeds and correspondingly increased electrical power signals. Thus, a desired acceleration period was associated with a unique running speed for a given system, and the downstream electrical components were, hopefully, economically matched to the electrical power signal from the alternator. The Malcolm and Levey et al. electrical components are described below in illustration of such prior art efforts.
In the Malcolm apparatus, the turbine is operated at a running speed of about 10,000 to 15,000 rpm, and the alternator produces about 15 volts, and this power signal is rectified. The rectified signal operates an oscillator which is operating at about 20 kilohertz at 12 volts. The oscillator has a square wave output which can be multiplied in a toroidal transformer to a value of about 2500 volts. This signal is then multiplied by a conventional cascade half-wave voltage multiplier of about 20 stages to produce a normal 50-55 kilovolt output. The cascade multiplier is a half-wave rectifier, and this oscillator-to-multiplier system is designed to produce the 55 kilovolt voltage as a DC voltage with a minimum of ripple voltage or peak.
It has been observed that the Malcolm circuitry just described, necessary for the conversion of low-voltage, low-frequency, e.g., 250 Hz at 12 volts, into high frequency and higher voltage, e.g., 20 KHz at 2500 volts, is subject to overheating and breakdown of the components when they have been miniaturized sufficiently for installation in a hand-manipulable spray apparatus. These latter problems are believed to be related to the fact that the air flow needed to provide the required acceleration in the Malcolm device results in a running speed and alternator voltage greater than desired. Consequently, the electrical restriction of the voltage to the desired 12-to-15 volts produces excess heat, which precludes continuous duty service without damage to the electrical components.
The Levey et al. spray apparatus is an improvement over the Malcolm apparatus and, in fact, adopts a significantly different operating approach since Levey et al. teach the use of a direct voltage having an alternating voltage ripple in excess of 15%. To that end, Levey et al. teach a simplified electrical apparatus which includes an air turbine operable from an external air supply at a speed in the order of 60,000 rpm, a directly coupled alternator to generate an alternating voltage in the order of 50 volts at about 1000 Hz, a step-up transformer connected to the alternator to transform the voltage thereof into a secondary voltage in the order of 2500 volts, and a long chain series voltage multiplier connected to the transformer to increase the voltage thereof to one in the order of 55-80 Kv.
The Levey et al. device resolves the overheating and breakdown problems in the Malcolm device substantially through elimination of certain of the electronic circuitry and incorporation of components of longer life which are less subject to premature failure. However, the Levey et al. device requires careful attention in respect to the physical dimensions of the turbine and alternator arrangement, as well as the pressure and volume flow of the driving air. The prior art energy conversion systems tend to be overly sensitive to variations in the supply of driving air. For example, moderate increases in pressure may cause overspeed of the turbine and alternator and corresponding excessive electrical loads that are damaging to the system. On the other hand, decreases in the pressure will result in unacceptably long acceleration periods before a suitable spray potential is applied to the electrode, as well as steady state operation at a lower than desirable potential with correponding decreases in the efficiency of the spraying. In addition to such variations in the pressure of the supplied air, the spray apparatus itself may be intermittently altered from an operating mode, wherein the delivered air flow is divided between a spray cap and the turbine, to a condition with the full air flow being delivered to the turbine.